Shape, Arrangement, and Size. Cocci (s., coccus) bacillus (pl., bacilli) 9/21/2013

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1 Shape, Arrangement, and Size Cocci (s., coccus) are roughly spherical cells. The other common shape is that of a rod, sometimes called a bacillus (pl., bacilli). Spiral-shaped procaryotes can be either classified as spirilla, which usually have tufts of flagella at one or both ends of the cell Cocci (s., coccus) They can exist as individual cells, but also are associated in characteristic arrangements that are frequently useful in their identification Diplococci (s., diplococcus) arise when cocci divide and remain together to form pairs. Long chains of cocci result when cells adhere after repeated divisions in one plane; this pattern is seen in the genera Streptococcus, Enterococcus, and Lactococcus Staphylococcus divides in random planes to generate irregular grapelike clumps S. aureus Staphylococcus aureus cocci arranged in clusters; colorenhanced scanning electron micrograph; average cell diameter is about 1 mm. S. agalactiae Streptococcus agalactiae, the cause of Group B streptococcal infections; cocci arranged in chains; color-enhanced scanning electron micrograph (4,800). Cocci (s., coccus) Members of the genus Micrococcus often divide in two planes to form square groups of four cells called tetrads. In the genus Sarcina, cocci divide in three planes producing cubical packets of eight cells. bacillus (pl., bacilli) Bacillus megaterium is a typical example of a bacterium with a rod shape Bacillus megaterium, a rod-shaped bacterium arranged in chains, Gram stain (600). 1

2 Vibrio Vibrios most closely resemble rods, as they are comma-shaped spirilla Spiral-shaped procaryotes can be either classified as spirilla, which usually have tufts of flagella at one or both ends of the cell, or spirochetes Vibrio cholera, curved rods with polar flagella; scanning electron micrograph Hyphomicrobium with hyphae and bud, Spiroplasma Spirochetes Spirochetes are more flexible and have a unique, internal flagellar arrangement. Actinomycetes Actinomycetes typically form long filaments called hyphae that may branch to produce a network called a mycelium pleomorphic some procaryotes are variable in shape and lack a single, characteristic form. Mycoplasma pneumoniae Bacteria vary in size as much as in shape Escherichia coli is a rod of about average size, 1.1 to 1.5 mm wide by 2.0 to 6.0 mm long Nanobacteria range from around 0.2 mm to less than 0.05 mm in diameter spirochaetes, which can reach 500 mm in length the photosynthetic bacterium Oscillatoria, which is about 7 mm in diameter (the same diameter as a red blood cell). 2

3 Bacterial Plasma Membrane THE CYTOPLASMIC MATRIX The Procaryotic Cytoskeleton The cytoplasmic matrix is the substance in which the nucleoid, ribosomes, and inclusion bodies are suspended. It lacks organelles bound by lipid bilayers (often called unit membranes), and is largely water (about 70% of bacterial mass is water). Until recently, it was thought to lack a cytoskeleton. The plasma membrane and everything within is called the protoplast; thus the cytoplasmic matrix is a major part of the protoplast. 3

4 Cytoskeleton Ribosomes Ribosomes are very complex structures made of both protein and ribonucleic acid (RNA). They are the site of protein synthesis Procaryotic ribosomes are smaller than the cytoplasmic or endoplasmic reticulum-associated ribosomes of eucaryotic cells. Procaryotic ribosomes are called 70S ribosomes (as opposed to 80S in eucaryotes), have dimensions of about 14 to 15 nm by 20 nm, a molecular weight of approximately 2.7 million, and are constructed of a 50S and a 30S subunit Ribosomes Procaryotic Ribosome. The two subunits of a bacterial ribosome are shown. The 50S subunit includes 23S rrna (gray) and 5S rrna (light blue), while 16S rrna (cyan) is found in the 30S subunit. A molecule of trna (gold) is shown in the A site. To generate this ribbon diagram, crystals of purified bacterial ribosomes were grown, exposed to X rays, and the resulting diffraction pattern analyzed Nucleoid The procaryotic chromosome is located in an irregularly shaped region called the nucleoid 30S subunit 50S subunit Nucleoid Chemical analysis of purified nucleoids reveals that they are composed of about 60% DNA, 30% RNA, and 10% protein by weight. In Escherichia coli, the closed DNA circle measures approximately 1,400 mm or about times longer than the cell Plasmids Many procaryotes (and some yeasts and other fungi) contain extrachromosomal DNA molecules called plasmids. Plasmids play many important roles in the lives of the organisms that have them. They also have proved invaluable to microbiologists and molecular geneticists in constructing and transferring new genetic combinations and in cloning genes, 4

5 Plasmids Plasmids are small, double-stranded DNA molecules that can exist independently of the chromosome. Both circular and linear plasmids have been documented, but most known plasmids are circular. Plasmids are able to replicate autonomously Bacterial Cell walls Bacterial Cell walls After Christian Gram developed the Gram stain in 1884, it soon became evident that most bacteria could be divided into two major groups based on their response to the Gram-stain procedure The gram-positive cell wall consists of a single 20 to 80 nm thick homogeneous layer of peptidoglycan (murein) lying outside the plasma membrane The gram-negative cell wall is quite complex. It has a 2 to 7 nm peptidoglycan layer covered by a 7 to 8 nm thick outer membrane. 5

6 Capsules, Slime Layers Some procaryotes have a layer of material lying outside the cell wall. This layer has different names depending on its characteristics. When the layer is well organized and not easily washed off, it is called a capsule It is called a slime layer when it is a zone of diffuse, unorganized material that is removed easily. When the layer consists of a network of polysaccharides extending from the surface of the cell, it is referred to as the glycocalyx Pili and Fimbriae Many bacteria have about 1-10 sex pili (s., pilus) per cell. These are hairlike structures that differ from fimbriae in the following ways. Pili often are larger than fimbriae (around 9 to 10 nm in diameter) Many procaryotes have short, fine, hairlike appendages that are thinner than flagella. These are usually called fimbriae. They can aid in attachment to objects Flagella Most motile procaryotes move by use of flagella (s., flagellum) Monotrichous bacteria (trichous means hair) have one flagellum; if it is located at an end, it is said to be a polar flagellum Amphitrichous bacteria (amphi means on both sides) have a single flagellum at each pole. lophotrichous bacteria (lopho means tuft) have a cluster of flagella at one or both ends Flagella are spread fairly evenly over the whole surface of peritrichous (peri means around) bacteria Filament Filament Hook Hook Pseudomonas monotrichous polar flagellation Spirillum lophotrichous flagellation L ring P ring P. vulgaris peritrichous flagellation S ring M ring gram-negative gram-positive 6

7 endospore. A number of gram-positive bacteria can form a special resistant, dormant structure called an endospore. Endospores develop within vegetative bacterial cells of several genera: Bacillus and Clostridium (rods), Sporosarcina (cocci), and others. These structures are extraordinarily resistant to environmental stresses such as heat, ultraviolet radiation, gamma radiation, chemical disinfectants, and desiccation Swollen sporangium Ribosomes Nucleoid Central Terminal Subterminal Core wall Cortex Spore coat Exosporium 7